Quantifying concentration distributions in redox flow batteries with neutron radiography

The continued advancement of electrochemical technologies requires an increasingly detailed understanding of the microscopic processes that control their performance, inspiring the development of new multi-modal diagnostic techniques. Here, we introduce a neutron imaging approach to enable the quantification of spatial and temporal variations in species concentrations within an operating redox flow cell. Specifically, we leverage the high attenuation of redox-active organic materials (high hydrogen content) and supporting electrolytes (boron-containing) in solution and perform subtractive neutron imaging of active species and supporting electrolyte. To resolve the concentration profiles across the electrodes, we employ an in-plane imaging configuration and correlate the concentration profiles to cell performance with polarization experiments under different operating conditions. Finally, we use time-of-flight neutron imaging to deconvolute concentrations of active species and supporting electrolyte during operation. Using this approach, we evaluate the influence of cell polarity, voltage bias and flow rate on the concentration distribution within the flow cell and correlate these with the macroscopic performance, thus obtaining an unprecedented level of insight into reactive mass transport. Ultimately, this diagnostic technique can be applied to a range of (electro)chemical technologies and may accelerate the development of new materials and reactor designs.

Figure S6: (a) Cumulative active species (TEMPO/TEMPO + ) concentration profiles over the electrode thickness at an inlet flow rate of 15.1 mL min -1 at applied potential steps of OCV, +0.3 V and +0.6 V. (b) Cumulative active species (TEMPO/TEMPO + ) and BF4 -supporting ion concentration profiles over the electrode thickness at an inlet flow rate of 15.1 mL min -1 at applied potential steps of OCV, +0.3 V and +0.6 V.

Electrolyte composition
The composition of the salt and active materials, as well as their combination, is important to ensure high selectivity with neutron radiography.In this work, we studied non-aqueous RFBs that typically use fluorinated anions such as hexafluorophosphate (PF6 -) or tetrafluoroborate (BF4 -).KPF6 is composed of elements having high neutron transmissions and therefore shows negligible neutron attenuation compared to the redox active molecules employed in this study (TEMPO/TEMPO + ).Hence, the low attenuation of KPF6 enables the selective imaging of redox active molecules, even in systems with complex reactive mass transport phenomena.On the contrary, BF4 -contains boron, which strongly attenuates the neutron beam.Therefore, when utilizing BF4 -as the supporting salt under white beam conditions, selective imaging of TEMPO/TEMPO + becomes challenging, resulting in the neutron image representing the sum of BF4 -+ TEMPO + TEMPO + concentrations.Accordingly, to guide the redox flow battery community in the right combinations of redox system and supporting salt, we listed the neutron attenuation with commonly used supporting salts and redox active molecules in Table S2.
Table S2: Neutron attenuation for coefficients of commonly used supporting salts and redox active molecules used in redox flow batteries to show their potential for neutron radiography.The effective neutron attenuation coefficients were calculated for both the NEUTRA and ICON beamlines using a concentration of 1 M in deuterated acetonitrile (corrected for the attenuation of deuterated acetonitrile assuming the molecules are fully dissolved, and the volume of the solution does not change).Moreover, not only the combination and composition of the salt and active materials is important, but also which specific imaging configuration is utilized.In our study, we employed two types of neutron imaging configurations: (1) white beam with thermal neutrons (NEUTRA beamline), and (2) cold neutrons coupled with the time-of-flight technique (ICON beamline).ICON offers neutron energy selective imaging by leveraging the energy-dependency of neutron attenuation in materials.This means that if the supporting salt and redox molecules have varying attenuations at different neutron energies, their concentrations can be distinguished, allowing for high imaging selectivity.On the other hand, NEUTRA relies on white beam imaging, resulting in a neutron image that reflects the total neutron attenuation of all mixed components in the beam path.Consequently, the electrolyte composition should depend on the imaging configuration of interest.

Figure S1 :Figure S4 :
Figure S1: Operando imaging of the active species transport in the NEUTRA beamline with the low attenuating KPF6 supporting salt at a lower flow rate.(a) Electrochemical sequence over time showing the applied potential steps and measured averaged current output at an inlet flow rate of 5.6 mL min -1 .(b-c) Cumulative active species (TEMPO/TEMPO + ) concentration profiles over the electrode thickness at an inlet flow rate of 5.6 mL min -1 .The averaged snapshots of the cell after image processing and the concentration profiles are shown for various applied potential steps: (b) -0.3 V and +0.3 V, and (c) -0.6 V and +0.6 V.

Figure S5 :
Figure S5: Capacity change over time for various applied potential steps and two inlet flow rates of 15.1 mL min -1 and 5.6 mL min -1 with the neutron attenuating BF4 -supporting ion in the NEUTRA beamline.

Figure S7 :
Figure S7: Electrochemical sequence over time showing the applied potential steps and measured current output at two inlet flow rates of 21.1 mL min -1 and 6.7 mL min -1 with the neutron attenuating BF4 -supporting ion in the ICON beamline.

Figure S8 :
FigureS8: Capacity change over time for various applied potential steps and two inlet flow rates of 21.1 mL min -1 and 6.7 mL min -1 with the neutron attenuating BF4 -supporting ion in the ICON beamline.

Table S1 :
Neutron attenuation coefficients of for commonly used reactor components to guide reactor design for neutron radiography experiments.The neutron attenuation coefficients were calculated for both the NEUTRA and ICON beamlines.